Layer Alignment in FinFET Fabrication

ABSTRACT

Methods for aligning layers more accurately for FinFETs fabrication. An embodiment method includes forming a first pattern in a workpiece using a first photomask, forming a second pattern in the workpiece using a second photomask, the second photomask aligned to the first pattern, and aligning a third pattern to the first and the second patterns by aligning a first feature of the third pattern to a first feature of the first pattern in a first direction, and aligning a second feature of the third pattern to a first feature of the second pattern in a second direction orthogonal to the first direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 13/217,702 filed on Aug. 25, 2011, entitled “Layer Alignment inFinFET Fabrication,” which application is hereby incorporated herein byreference.

BACKGROUND

The present disclosure relates generally to semiconductor devices, andmore specifically to the fabrication process of Fin field effecttransistors (FinFETs) and methods of aligning approach more accuratelyduring the fabrication process of FinFETs.

There are significant pressures on the semiconductor industry to enablesmaller and smaller critical dimensions of integrated circuits. Finfield-effect transistors (FinFET) have smaller device sizes while withincreased channel widths, which channels include the channels formed onthe sidewalls of the fins and the channels on the top surfaces of thefins. To maximize the channel width of a FinFET, the FinFET may includemultiple fins, with the ends of the fins connected to a same source anda same drain.

The fabrication process of integrated circuits (ICs) in general andFinFETs in particular include several stages, of which, the definitionof a pattern associated with the circuit is of critical importance. Thepattern may then be fabricated on a substrate using photolithographyprocesses. Photolithographic methods typically include the use ofsuccessive resist layers that are latently imaged and subsequentlydeveloped and patterned over a substrate for purposes of fabricating anyof several structures within the substrate. Successful semiconductorfabrication requires highly accurate alignment of features on masks usedin photolithographic processes, and of their projection onto the wafer,such that successive mask-defined patterns of material are located onthe wafer with accuracy in the low tens of nanometers range. Thealignment process is never perfect; however, the overlay alignmentmeasurement is critical for FET operation and must be tightly andmeasurably controlled during manufacture.

The front-end-of-line process (FEOL) denotes the first portion of ICmanufacturing where the individual devices are patterned in thesemiconductor. FEOL generally covers everything up to (but notincluding) the deposition of metal interconnect layers, which containsall processes of CMOS fabrication needed to form fully isolated CMOSelements. For example, a FEOL piece may include a semiconductorsubstrate, gates, source/drain regions, isolation regions, spacers,contacts, dielectric material, and first level metal interconnects.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 is a flow chart illustrating an embodiment of a method ofaligning FEOL layers to the third pattern in X direction and to thefirst pattern in Y direction;

FIGS. 2( a)(i)(ii)(iii) are various views of a first pattern associatedwith the method of FIG. 1. FIGS. 2( a)(ii)(iii) are a top view and across-sectional view respectively of an embodiment of a substratecorresponding to the pattern of FIG. 2( a)(i);

FIGS. 2( b)(i)(ii)(iii) are various views of a second pattern associatedwith the method of FIG. 1. FIGS. 2( b)(ii)(iii) are a top view and across-sectional view respectively of an embodiment of a substratecorresponding to the pattern of FIG. 2( b)(i);

FIGS. 2( c)(i)(ii)(iii) are various views of a third pattern associatedwith the method of FIG. 1. FIGS. 2( c)(ii)(iii) are a top view and across-sectional view respectively of an embodiment of a substratecorresponding to the pattern of FIG. 2( c)(i); FIG. 2( d) is a top viewof the third pattern on top of the fins patterned following the firstpattern to define an active area; FIG. 2( e) is a top view of the activearea; FIG. 2( f) is a top view a poly on active layout in devicecircuit, which active layer patterned in FIG. 2( e);

FIG. 3( a) is a top view of the third pattern on top of fins to definean active area in device circuit with a different shape; FIG. 3( b) is atop view of the active area; FIG. 3( c) is a top view of a poly layer onactive layer patterned in FIG. 3( b);

FIG. 4( a) is a top view of the third pattern on top of fins to definean overlay outer alignment mark; FIG. 4( b) is a top view of the outeroverlay alignment mark in active layer following the first and thirdpattern process; FIG. 4( c) is a top view of a poly layer(FEOL) inneroverlay alignment mark align to active layer which has the same positioncenter;

FIG. 5( a) is a top view of the third pattern on top of fins to defineanother alignment mark, wherein the third pattern covers some protectedareas in Y-direction and cut in X-direction; FIG. 5( b) is a top view ofthe active outer overlay alignment mark; and FIG. 5( c) is a top view ofa poly layer (FEOL) inner overlay alignment mark align to active layerwhich has the same position center; and

FIGS. 6( a)-6(c) illustrate one type of alignment marks used by exposuretools manufactured using processes illustrated in FIGS. 2( a) to 2(c)process.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and forming of the present exemplary embodiments arediscussed in detail below. It should be appreciated, however, thatembodiments of the present disclosure provide many applicable inventiveconcepts that can be embodied in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificways to make and use the disclosure, and do not limit the scope of thedisclosure.

The present disclosure will be described with respect to exemplaryembodiments in a specific context, namely fabrication processes of Finfield effect transistors (FinFETs) and methods of aligning layers moreaccurately during the fabrication process of FinFETs.

Referring to FIG. 1, a flow chart illustrating a method 100 forfabricating a FinFET, or portion thereof, is provided. The method beginsat step 102 where a first pattern is provided. The first pattern maydefine a configuration of dummy line structures (or features) which areused to form spacer elements (e.g., abutting the line structure). A“pattern”, as described throughout the disclosure, may be provided in alayout design file (e.g., GDSII file), a photomask, formed on asubstrate, and/or in other suitable forms.

Referring to the example of FIGS. 2( a)(i), 2(a)(ii), 2(a)(iii), a firstpattern is illustrated. The first pattern is exemplary only and notintended to be limiting, any configuration and quantity of elementsproviding a pattern is possible.

The first pattern includes a plurality of line elements determined froma layout design file and formed on a photomask 202 shown in FIG. 2(a)(i). FIG. 2(a)(iii) illustrates a substrate 203 with a pattern formedthereon following the photomask 202, while FIG. 2( a)(ii) illustratesthe top view of the pattern formed shown in FIG. 2( a)(iii). The patternshown in FIGS. 2( a)(ii) and 2(a)(iii) are formed using photolithographywhich exposes the substrate 203 to the mask 202. The photomask 202 maybe an attenuating phase shift mask (att-PSM), an alternating PSM(alt-PSM), a chromeless phase shift mask (CPL), and/or other suitablemask types. The substrate 203 may comprise a bulk semiconductor wafer oran upper single crystal silicon layer separated from a lowersingle-crystal silicon layer by a buried oxide (BOX) layer.

An optional dummy layer 204 may be disposed on the substrate 203. Thedummy layer 204 may include amorphous carbon, polysilicon, and/or othersuitable material. Numerous other embodiments and layers may bepossible. A plurality of hard mask layers 205 and 206 are disposed onthe dummy layer 204. In an embodiment, hard mask layers comprise aplurality of layers formed of different materials, e.g., siliconnitride. Optionally, amorphous carbon layer may be formed over a siliconnitride layer. Plasma enhanced (PE) oxide, which may be a silicon oxideformed using plasma enhanced chemical vapor deposition (PECVD), isformed over the amorphous carbon layer. Hard mask layers may alsoinclude additional layers (not shown) comprising, but not limited to, anadditional amorphous layer over silicon oxynitride layer, an additionalsilicon oxynitride layer over the additional amorphous carbon layer,and/or an additional bottom anti-reflective coating (ARC) over theadditional silicon oxynitride layer.

Another dummy layer may be patterned to form a plurality of dummy linefeatures 207 having the first pattern 202. The dummy line features 207may include amorphous carbon, polysilicon, and/or other suitablematerial. A spacer layer may be formed on the dummy line features 207 toform spacer elements 208 abutting the dummy line features 207. Thespacer elements 208 may include silicon oxide, silicon nitride, siliconoxynitride, and/or other suitable materials. The spacer elements 208 maydefine a critical dimension of an element to be formed on the substrate203. For example, in an embodiment the spacer elements 208 will define awidth and a pitch for elements formed. Thus, the first pattern providesdummy line features 207 which allow for adjacent spacer elements 208 tobe formed, based on which the FINs will be formed.

Referring back to FIG. 1, the method 100 then proceeds to step 104 wherea second pattern is provided. The second pattern may remove a portion ofspacer elements 208 overlying dummy line features defined by the patternof step 102. FIGS. 2( b)(i), 2(b)(ii), 2(b)(iii), illustrate an example.In FIG. 2( b)(i), a pattern is formed on a photomask 212. The photomask212 may be substantially similar to the photomask 202, described abovewith reference to FIG. 2( a)(i). FIG. 2( b)(ii) and FIG. 2( b)(iii)illustrate a top view and a cross-sectional view of a pattern formed onthe substrate 203 by photolithography process following the photomask212. The second pattern is formed by photolithography processes usingthe photomask 212, which defines portions of spacer elements 208 to beremoved and a plurality of dummy line features 207 to be removed. As theresult, a plurality of spacer elements 213 are formed, which may definea pitch/width for a FIN of FinFET.

Referring back to FIG. 1, the method 100 then proceeds to step 106 wherea third pattern is provided. The third pattern may be known as a cutpattern, a cut and protect pattern, or an OD pattern (e.g., defining theactive area). The third pattern may remove unwanted spacer elementsand/or ends of spacer elements previously formed using the patterns ofsteps 102 and 104. FIGS. 2( c)(i), 2(c)(ii), 2(c)(iii), illustrate anexample. In FIG. 2( c)(i), a pattern is illustrated on a photomask 222.The photomask 222 may be substantially similar to the photomask 202,described above with reference to FIGS. 2( a)(i)-(iii). FIGS. 2( c)(ii),2(c)(iii) illustrate a top view and a cross-sectional view of a pattern222 provided on the substrate 203. The pattern 222 is formed on thesubstrate 203 by photolithography processes exposing the substrate 203to the pattern 222. Thus, the pattern 222 includes spacer elementshaving portions at end of the dummy line features removed. One or morespacer elements have also been removed from the substrate 203 (e.g.,those falling outside the defined active area). Additional steps (notshown in FIG. 1) may be taken to form a grid of FINs on the substrate203, including: using the pattern 222 as a masking element, patterningthe hard mask layers 205 and 206 using the spacer pattern 223, andpatterning the dummy layer 204, which may be further used as a maskingelement for forming FINs in the substrate 203. Therefore, the pattern223 includes a grid of FINs formed on the substrate 203 (not shown inthe figure) in a selected area (e.g., active area), wherein the selectedarea is defined by the third pattern.

FIG. 2( d) is a top view of the third pattern 222, i.e., the patternsformed from the overlay of the first, second, and third patternsdescribed above. The number of 214 FINs and the size of the cut patterndefining an active area are all for illustrative purposes only. Theremay be different number of FINs and the 222 cut pattern may be ofdifferent sizes and located in different positions on top of 214 Fins.The cut pattern 222 in FIG. 2( d) is a rectangle. Other shapes of cutpatterns are possible to be used as well. FIG. 2( e) is a top view ofthe active area resulted from applying photolithography process to theoverlay of patterns shown in FIG. 2( d), where 223 shows a plurality ofspacer elements in active area of the device.

After an active area is defined following the overlay of the first, thesecond, and the third patterns, the method 100 then proceeds to step 108where additional FEOL layers may be formed on top of the first threelayers. Additional FEOL layers may include contact layers, poly layers,and depositing a shield layer, or the like.

FIG. 2( f) is a top view of a poly layer on a FinFET active OD. In the Xdirection, the poly layer 233 is aligned to the edge of the active areadefined by the third pattern. In the Y direction, the poly layer 233 isfurther aligned to the edge of fins defined by the spacer elementsfollowing the first pattern. The position and the size of the poly layer233 are only for illustrative purposes. Other positions and sizes of thepoly layer 233 can be used. FEOL layers other than a poly layer can beused too. Those skilled in the art can readily see many other variationswhile remaining within the scope of the present disclosure. The X+ΔXdimension is defined by the third pattern, extended over the edge of thepoly layer 233. The Y+ΔY dimension is defined to be the distance of thepoly layer 233 extended over the edge of the FIN, defined by the firstpattern 202 as shown in FIG. 2( a)(i). The measurements X and Y are thedesired position the poly layer 233, while ΔX and ΔY are the overlaymisalignment values compared to the desired locations. The total amountY+ΔY measures the device overlay (POLY to active layer) performance in Ydirection. The total amount X+ΔX measures device overlay (POLY to activelayer) performance in X direction.

Alternatively, instead of using a rectangle cut pattern 222 to define anactive area as shown in FIG. 2( d), other shapes of cut patterns can beused in step 106 of the method 100. An illustrative embodiment using apolygon with 6 sides as the cut pattern is shown in FIG. 3( a). FIG. 3(a) is a top view of the third pattern on top of the fins 214 to definean active area, wherein the third pattern is of a shape of a polygonwith 6 sides. There may be different number of FINs and the 222 cutpattern may be of different sizes and located in different positions ontop of 214 Fins. FIG. 3( b) is a top view of the active area resultedfrom applying photolithography process to the overlay of patterns shownin FIG. 3( a).

After an active area is defined following the overlay of the first, thesecond, and the third patterns, the method 100 then proceeds to step 108where additional FEOL layers may be deposited on top of the first threepatterns. Additional FEOL layers may include contact layers, polylayers, a shield layer, or the like. FIG. 3( c) is a top view of a polylayer aligned to an active FinFET layer. In X direction, the poly layer233 is aligned with the active area defined by the third pattern. In Ydirection, the poly layer 233 is further aligned with the Fins definedby the spacers following the first pattern, similar to the pattern shownin FIG. 2( a)(i). The X+ΔX dimension is defined by the third pattern,extended over the edge of the poly layer 233. The Y+ΔY dimension isdefined to be the distance of the poly layer 233 extended over the edgeof the FIN, defined by the first pattern 202. The measurements X andYare the desired position the poly layer 233, while ΔX and ΔY are theoverlay misalignment values compared to the desired locations. The totalamount Y+ΔY measures the device overlay (POLY to active layer)performance in Y direction. The total amount X+ΔX measures deviceoverlay (POLY to active layer) performance in X direction. The positionand the size of the poly layer 233 are only for illustrative purposes.Other positions and sizes of the poly layer 233 can be used. FEOL layersother than a poly layer can be used too. Those skilled in the art canreadily see many other variations while remaining within the scope ofthe present disclosure.

In order to align FEOL layers as shown in FIGS. 2 and 3 more accurately,alignment marks can be used to measure the position error of aligningFEOL layers in step 106 of the method 100 shown in FIG. 1. Anillustrative embodiment using an overlay alignment mark with a cutpattern 222 in a monitor pad area is shown in FIG. 4( a). FIG. 4( a) isa top view of the third pattern on top of the fins 214 in a monitor padto define an outer overlay alignment mark 234, which is a monitor pad ofan overlay box, wherein the third pattern 222 is of a shape of rectanglewith inside area uncovered. There may be different number of FINs andthe 222 cut pattern may be of different sizes and located in differentpositions on top of 214 Fins. FIG. 4( b) is a top view of the outeroverlay alignment mark after the third pattern process to define theactive outer overlay mark for following FEOL alignment.

After the outer overlay alignment mark is defined following the overlayof the first, the second, and the third patterns, the method 100 in FIG.1 then proceeds to step 108 where additional FEOL layers may bedeposited on top of the first three layers. Additional FEOL layers mayinclude contact layers, poly layers, and depositing a shield layer, orthe like. FIG. 4( c) is a top view of an inner poly alignment boxpattern to align a poly layer, which is an additional FEOL layer, on topof the first three layers. In X direction, the poly layer 233 is alignedwith the alignment mark defined by the third pattern shown in FIG. 4(a). In Y direction, the poly layer 233 is further aligned with thealignment mark defined by the spacers following the first pattern,similar to the one shown in FIG. 2( a)(i). The X+ΔX dimension is definedby the third pattern, extended over the edge of the poly layer 233. TheY+ΔY dimension is defined to be the distance of the poly layer 233extended over the edge of the alignment mark, defined by the firstpattern 234. The measurements X and Y are the desired position the polylayer 233, while ΔX and ΔY are the resulted shifting values compared tothe desired locations. The position and the size of the poly layer 233are only for illustrative purposes. Other positions and sizes of thepoly layer 233 can be used. FEOL layers other than a poly layer can beused too. Those skilled in the art can readily see many other variationswhile remaining within the scope of the present disclosure. The outeroverlay alignment mark design described herein can reflect the deviceoverlay more accurately, and it can further prevent the process residueor over correction effect.

In FIG. 4( c), the ΔX is defined to be the overlay shift of the polylayer 233 to the edge of the alignment mark in the horizontal direction,defined by the third pattern shown in FIG. 4( a). The ΔY is defined tobe the overlay shift of the poly layer 233 to the edge of the alignmentmark in the vertical direction, defined by the first pattern, similar tothe one shown in FIG. 2( a)(i). ΔX and ΔY are measurements defining theintended overlay shift position of the poly layer 233 in device circuit.By aligning the FEOL layer (e.g., 233 poly layer) to the outer overlayalignment mark defined by the third pattern mask, the FEOL layout canget the real active area position and accurately reflect the deviceperformance.

Alternatively, instead of aligning FEOL layers as shown in FIG. 4, othershapes of alignment marks can be used to align FEOL layers in step 106of the method 100. An illustrative embodiment using an alignment markwith a cut pattern 222 is shown in FIG. 5( a). FIG. 5( a) is a top viewof the third pattern on top of the fins to define an alignment mark 234,wherein the third pattern 222 is of a shape of an open rectangle areauncovered. Moreover, the cut pattern 222 covers a protected area 235where the Fins in area 235 should be protected. There may be differentnumber of FINs and the 222 cut pattern may be of different sizes andlocated in different positions on top of 214 Fins. The protected area235 may be in Y direction rather than in X direction. FIG. 5( b) is atop view of the alignment mark resulted from applying photolithographyprocess to the overlay of patterns shown in FIG. 5( a).

After the alignment mark is defined following the overlay of the first,the second, and the third patterns, the method 100 shown in FIG. 1 thenproceeds to step 108 where additional FEOL layers may be deposited ontop of the first three layers. Additional FEOL layers may includecontact layers, poly layers, a shield layer, or the like. FIG. 5( c) isa top view of an alignment method to align a poly layer, which is anadditional FEOL layer, on top of the first three patterns. In Xdirection, the poly layer 233 is aligned with the active area defined bythe third pattern. In Y direction, the poly layer 233 is further alignedwith the alignment mark defined by the spacers following the firstpattern, similar to the one shown in FIG. 2( a)(i). The position and thesize of the poly layer 233 are only for illustrative purposes. Otherpositions and sizes of the poly layer 233 can be used. FEOL layers otherthan a poly layer can be used too. Those skilled in the art can readilysee many other variations while remaining within the scope of thepresent disclosure.

Similarly to using alignment marks to measure the position error to helpto align FEOL layers more accurately as shown in FIGS. 4( a)-4(c) andFIGS. 5( a)-5(c), overlay marks can be used to achieve a similarpurpose. FIGS. 6( a)-6(c) are such an illustrative example, where theoverlay marks are used by metrology tools while the alignment marks areused by exposure tools. FIG. 6( a) is a top view of an FIN pattern 614defined by the first pattern comprising a plurality of FIN lines. FIG.6( b) is a top view of the cut pattern using in the third patternprocess to define the overall overlay mark shown in FIG. 6( c) forfollowing FEOL alignment. There may be different number of FINs 614 andthe 622 cut pattern may be of different sizes and located in differentpositions on top of 614 FINs. After the overlay mark is definedfollowing the overlay of the first, the second, and the third patterns,the method 100 in FIG. 1 then proceeds to step 108 where additional FEOLlayers may be deposited on top of the first three layers. AdditionalFEOL layers may include contact layers, poly layers, and depositing ashield layer, or the like.

FIG. 6( c) is a top view of an alignment mark of exposure tools 634. TheX direction edge of the alignment mark is defined by the cut pattern622, which is the third pattern, while the Y direction edge of thealignment mark is defined by the FIN pattern 614 which is the firstpattern. The overall alignment in X and Y directions are X+ΔX and Y+ΔYrespectively. The alignment X and Y are the desired position, while ΔXand ΔY are the resulted shifting values compared to the desiredlocations. Note that the combined patterns form a gradient in the Ydirection. Such a gradient can be used for machine vision patterning forexposure aligned, as is well known. A like pattern as shown in FIG. 6(c), but rotated 90° (pattern 614 and 622) can be used to form a machinevision recognizable pattern in the X direction.

The flow chart illustrating a method 100 in FIG. 1 for fabricating aFinFET, or portion thereof, is exemplary only and not intended to belimiting. There are many other possible variations where those skilledin the art can readily see while remaining within the scope of thepresent disclosure. An alternative method may comprise: forming a firstpattern in a workpiece using a first photomask, such as a patterndefines a plurality of spacer elements abutting a plurality of dummyline features, as shown in FIG. 2( a); forming a second pattern in theworkpiece using a second photomask, which is aligned to the firstpattern, such as a pattern defines an active area or an alignment markas shown in FIG. 2( d) or FIG. 4( a); aligning a third pattern to thefirst and second patterns by (i) aligning a first feature of the thirdpattern to a first feature of the first pattern in an Y direction, and(ii) aligning a second feature of the third pattern to a first featureof the second pattern in an X direction. Embodiments of the thirdpattern may comprise FEOL layers such as metal layers, poly layers, orshield layers, or the like, as shown in FIG. 2( f). In the X direction,aligning a first feature of the third pattern to a first feature of thefirst pattern is defined to be the overlay shift ΔX of the poly layer233 to the edge of the alignment mark, defined by the third patternshown in FIG. 4( a). In the Y direction, aligning a second feature ofthe third pattern to a first feature of the second pattern is defined tobe the overlay shift ΔY of the poly layer 233 to the edge of thealignment mark, defined by the first pattern, similar to the one shownin FIG. 2( a)(i). X+ΔX and Y+ΔY are measurements defining the resultingposition of the poly layer 233.

What is claimed is:
 1. A method, comprising: forming a first pattern ina workpiece using a first photomask; forming a second pattern in theworkpiece using a second photomask, the second photomask aligned to thefirst pattern; and aligning a third pattern to the first and the secondpatterns by aligning a first feature of the third pattern to a firstfeature of the first pattern in a first direction, and aligning a secondfeature of the third pattern to a first feature of the second pattern ina second direction orthogonal to the first direction.
 2. The method ofclaim 1, wherein the third pattern defines a FEOL layer, the FEOL layerone of a shield layer and a poly layer.
 3. The method of claim 1,wherein the second pattern defines an active area.
 4. The method ofclaim 1, wherein the second pattern defines an alignment mark.
 5. Themethod of claim 1, wherein the first pattern defines a plurality ofspacer elements abutting a plurality of dummy line features.
 6. Themethod of claim 1, wherein the third pattern is formed by aphotolithography process.
 7. The method of claim 1, wherein the thirdpattern includes spacer elements having end portions removed.
 8. Themethod of claim 1, wherein the third pattern includes a grid of fins. 9.The method of claim 1, wherein the third pattern is formed using a thirdphotomask.
 10. A method, comprising: forming a first pattern in aworkpiece using a first photomask; forming a second pattern in theworkpiece using a second photomask, the second photomask aligned to thefirst pattern; forming a third pattern using a third photomask; andaligning a first feature of the third pattern to a first feature of thefirst pattern in a first direction, and aligning a second feature of thethird pattern to a first feature of the second pattern in a seconddirection orthogonal to the first direction.
 11. The method of claim 10,wherein the third pattern defines a FEOL layer, the FEOL layer one of ashield layer and a poly layer.
 12. The method of claim 10, wherein thefirst pattern defines a plurality of spacer elements abutting aplurality of dummy line features.
 13. The method of claim 10, whereinthe second pattern defines one of an active area and an alignment mark.14. The method of claim 10, wherein the third pattern is formed by aphotolithography process.
 15. The method of claim 10, further comprisingtrimming away end portions of spacer elements in the third pattern. 16.The method of claim 10, wherein the third pattern defines a plurality offins.
 17. A method, comprising: forming a first pattern in a workpieceusing a first photomask, the first pattern defining a plurality ofspacer elements abutting a plurality of dummy line features; forming asecond pattern in the workpiece using a second photomask, the secondpattern defining an active area, the second photomask aligned to thefirst pattern; and aligning a third pattern to the first and the secondpatterns by aligning a first feature of the third pattern to a firstfeature of the first pattern in a first direction, and aligning a secondfeature of the third pattern to a first feature of the second pattern ina second direction orthogonal to the first direction.
 18. The method ofclaim 17, wherein the third pattern is formed using a third photomask.19. The method of claim 18, wherein at least one of the first photomask,the second photomask, and the third photomask is one of an attenuatingphase shift mask, an alternating phase shift mask, and a chromelessphase shift mask.
 20. The method of claim 17, wherein at least one ofthe first pattern, the second pattern, and the third pattern are formedby way of a photolithography process.